Hi- I'm trying to use dark subtract to atmospherically correct some high-res images I have. I'm having a few issues: 1) My files are rather large, in an .ntf format. When I enter them into the workflow, my ENVI window basically stops functioning and I have to force quit. Do the files need to be in a certain format, or under a certain size to work? I did a spatial subset of my image, about 1/5 of the actual size, and it worked as a .tif but not as an .ntf so I'm guessing .ntf isn't a compatible format. 2) My image is so large that I cannot convert it all into a .tif file. Are there any other compatible file types that I could try that might allow me to convert the entire image? 3) My image has an area of "no data" values around it, but when I enter the file into the workflow, there is no "data ignore value" field, as ENVI help claims there to be. Does anyone know what's going on here? 4) Lastly, when I ran the .tif clip through the workflow and it completed successfully, I compared it to the original, un-corrected clip and saw no difference. Is this normal, or did something go wrong? I'm new to atmospheric correction, and could use all the help I can get :) Thanks! UPDATE: I've figured out the issues with the file type (questions 1 and 2, above). However, doing a pixel-by-pixel examination of the output, the image is exactly the same as the uncorrected input file. I'm pretty sure this is related to the "no data" area around the image, and the fact that I can't opt out of those pixels (question number 3, above). So essentially all I need to figure out now is how to get those pixels to be discounted in the dark subtract process. Does anyone know how to do this? Thanks.